Memory frame architecture for instruction fetches in simulation

ABSTRACT

A simulation technique that handles accesses to a frame of instruction memory by inserting a command object between a frame proxy and a memory frame provides improved throughput in simulation environments. The instruction frame, if present, processes the access to the frame. If an instruction frame is not present for the accessed frame, the memory frame handles the request directly. The instruction frame caches fetched and decoded instructions and may be inserted at the first access to a corresponding instruction memory frame. The instruction frame can track write accesses to instruction memory so that changes to the instruction memory can be reflected in the state of the instruction frame. Additional check frames may be chained between the interface and the memory frame to handle breakpoints, instruction memory watches or other access checks on the instruction memory frame.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to memory access management, and inparticular, to an architecture using a memory frame proxy for accessesto instruction memory in a simulation environment.

2. Description of Related Art

Simulators, debuggers and accelerators that can be implemented inhardware, software or a combination of both, are typically used inverification of program code as well as verification of designs ofprocessor integrated circuits (ICs) and other large-scale logic.

The simulation of instruction fetches and decodes is a compute-intensivetask, since each time an access to instruction memory is made bysimulated pipeline, if the instruction must be fetched from thesimulated instruction memory, the instruction must be decoded, andchecks must be performed if there are any associated with theinstruction location or operands implicated by the instruction.

Caching of simulations of instruction memory is not a viable techniquewhere instruction memory may be moved by another process, or whereself-modifying code may alter the values stored in instruction memory.Therefore, the fetching and decoding of instructions represents aserious bottleneck in simulation throughput and it is desirable toimprove the performance simulators, debuggers and the link.

BRIEF SUMMARY OF THE INVENTION

The invention is embodied in a computer system and computer programproduct that simulate accesses to instruction memory. The computersystem is a computer system executing program instructions for carryingout the method and the computer program product is a program forcarrying out the method.

The method simulates accesses to a frame of instruction memory by usingproxy frames to receive accesses to instruction memory frames in amemory subsystem. An instruction frame can be inserted in a linked listbetween a proxy frame and the corresponding memory frame, along withother check frames such as breakpoint frames, to handle processing ofinstruction fetches and other accesses to the corresponding instructionmemory frame. If an instruction frame is not present for the accessedframe, the memory frame handles the request directly. The instructionframe may be inserted at the first access to the instruction memoryframe and may cache fetched and decoded instructions. The instructionframe can track write accesses to instruction memory so that changes tothe instruction memory can be reflected in the state of the instructionframe. Additional check frames may be chained between the interface andthe memory subsystem to handle breakpoints, instruction memory watchesor other accesses to the instruction memory frame. As executionproceeds, the instruction frame may store pointers to a next decodedinstruction from a given non-branch instruction so that completion ofprocessing does not require determining a next instruction to execute,nor re-entry of the instruction frame.

The foregoing and other objectives, features, and advantages of theinvention will be apparent from the following, more particular,description of the preferred embodiment of the invention, as illustratedin the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asa preferred mode of use, further objectives, and advantages thereof,will best be understood by reference to the following detaileddescription of the invention when read in conjunction with theaccompanying Figures, wherein like reference numerals indicate likecomponents, and:

FIG. 1 is a pictorial diagram depicting a simulation architecture inwhich the techniques disclosed herein can be practiced in accordancewith embodiments of the invention.

FIG. 2A and FIG. 2B are pictorial diagrams of simulated instructionmemory access paths in the simulation architecture of FIG. 1.

FIG. 3A is a pictorial diagram depicting an architecture of instructionframe 18 of FIGS. 2A-2B and FIG. 3B is a pictorial diagram depicting anarchitecture of check frame 18A of FIG. 2B.

FIG. 4 is a flow chart depicting a method of simulating instructionmemory accesses in accordance with an embodiment of the presentinvention.

FIG. 5 is a block diagram illustrating a computer system in whichprogram code according to an embodiment of the present inventionimplementing a method according to an embodiment of the invention isexecuted.

DETAILED DESCRIPTION OF THE INVENTION

The present invention encompasses computer-performed methods andprograms for simulating execution of program code by a processor.Accesses to instruction memory are made from a memory interface frameproxy, which may be an object, a function or another code entry point atwhich the simulator obtains the value of a location (generally aninstruction or op-code) in instruction memory in order to simulateexecution of the instruction. An instruction frame may be insertedbetween the frame proxy and the instruction memory frame, which isgenerally a data page managed by the simulation program that containsthe code for the memory frame being simulated. For the purposes ofillustration, it can be assumed the frame size, the page size of thesimulating system and the page size of the system being simulated arethe same, but differences in page size can be accommodated byappropriate mapping in the simulator and the frame size may be multiplepages or multiple frames can be provided per page.

An instruction frame is generally inserted in the chain between theframe proxy and the instruction memory frame at a first access to aninstruction memory frame and may be retired as needed to conserveresources in the simulator, for example on a most-recently-accessedbasis. The instruction frame stores fetched and decoded instructions sothat fetching and decoding does not have to be repeated for eachexecution of a given instruction, while maintaining coherency in theface of self-modifying code or other operations that might corrupt theunderlying instruction memory frame, since all accesses to theinstruction memory frame will be processed by the instruction frame onceit is inserted between the frame proxy and the memory frame. One or morecheck frames may be inserted for a given memory frame, the arrangementof which can be maintained via a linked list. Functions such asbreakpoints or instruction memory watches can be implemented and easilyinserted and removed by inserting independent check frames correspondingto those functions.

Referring now to FIG. 1, a simulation architecture is shown, in whichtechniques according to embodiments of the invention can be practiced. Asimulator 10, i.e., a simulation program, implements a memory interface12, through which code storage of a simulated memory subsystem 14 isaccessed. Simulated memory subsystem 14 is controlled by simulator 10that contains stored values that represent the contents of memory thatis being simulated along with one or more simulated processors thataccess the memory. The simulated memory may be data and instructionmemory combined, but what is of concern in the present example isinstruction memory storage. Memory interface 12 is illustrated as agetcode (&next_instruction) interface that corresponds to an instructionfetch, which is performed by a simulated processor. In the presentinvention, as will be illustrated in further detail below, a frame proxycan implement memory interface 12, and one or more command and checkframes can be chained between memory interface 12 and simulated memorysubsystem 14, so that simulation can be accelerated and breakpoints,watches, protection and other mechanisms that require special handlingcan be implemented. Each frame of memory simulated by simulated memorysubsystem 14 can have a frame proxy and one or more special handlingframes chained between memory interface 12 and memory frames insimulated memory subsystem 14, which generally correspond to memorypages in the simulated computer system, but that may be alternativelylarger or smaller than a page. The per-frame insertion of frame proxiesprovides direct access for frames that have no special handling, so thatthroughput is maximized, because no special checks are required todetermine if special handling is required for that frame.

Referring now to FIG. 2A, a simulated memory access path for aninstruction memory frame within the simulation environment of FIG. 1 isshown. Each frame of simulated instruction memory subsystem 14 has acorresponding entry to a linked list that may point directly to aninstruction memory frame 18 within simulated instruction memorysubsystem 14, or that may contain a linked-list of one or more handlingframes headed by a frame proxy 16. The illustrated access path is foronly one particular frame of memory; other memory frames have their ownlinked list of proxy objects. Frame proxy 16 receives requests foraccess to a particular frame of simulated instruction memory subsystem14, i.e., instruction fetch requests, and generally handles thoserequests by storing previously fetched and decoded instructions withinan instruction frame 18A. The instructions are stored within instructionframe 18A so that instructions do not have to be fetched from simulatedinstruction memory subsystem 14 and decoded each time the instructionsare executed. Instruction frame 18A is generally inserted in the chainbetween proxy frame 16 and instruction memory frame 18 at the firstfetch of an instruction stored in instruction memory frame.

Referring to FIG. 2B, an access path for a memory frame for whichanother special handling frame: check frame 18B has been insertedbetween frame proxy 16 and instruction frame 18A. Check frame 18Bperforms special handling for memory frame 18 such as checks, e.g.,breakpoint handling, then passes the access request through toinstruction frame 18A, which provides a decoded program instruction andnext instruction pointer result for non-branch instructions. In each ofthe cases shown in FIG. 2A and FIG. 2B, access to simulated memorysubsystem 14 may or may not be required, since if instruction frame 18Ais present and the instruction has been previously executed, a decodedinstruction corresponding to the program instruction at the requestedmemory location is cached at a location referenced in instruction frame18A.

Referring now to FIG. 3A, an architecture of instruction frame 18A ofFIGS. 2A-2B is shown in accordance with an embodiment of the presentinvention. Instruction frame 18A is not a breakpoint, watch orprotection object, and thus contains decoded program instructionsaccording to the examples given herein. As instruction frame 18A ispopulated with decoded non-conditional-branch program instructions,entries are placed in table 24, which may be generated according to thefollowing example:

Location Decoded instruction Next instruction entry Address1 Decode 1&next_instruction1 (&branch_to1) Address2 Decode 2 &next_instruction2(&branch_to2) Address 3 Decode 3 &next_instruction2 (&branch_to3)The next instruction pointers are generally indices or pointers to otherentries in table 24, which are generally populated in advance for theinstruction frame, so that storage for decoded instructions does nothave to be allocated on-the-fly and the next instruction pointers arealready initialized. Once the instruction frame and associated storageare set up, as the simulation proceeds, instructions are decoded and theresults cached. If the instructions have already been decoded, theprevious decoding results are available for use without reference to theunderlying memory. Next instruction reference 26A illustrates a nextinstruction reference that points to another entry in table 24. Alsoincluded in table 24 for branch instructions, is a “branch-to” pointer26B which points to an entry in table 24 or another location, at which adecoded instruction for the non-sequential next instruction executedwhen the branch condition is true. The memory frame has a run( ) methodthat is valid for both decoded instructions and “dynamic” instructions(instructions that have not been decoded). Both types of instructionobjects implement the run( ) method, so that once a pointer to theobject representing the instruction is obtained, i.e., via thenext_instruction pointer, the run( ) method can be invoked with nopenalty for checking for a decoded instruction. The availability of thenext_instruction pointer for all cases saves a fetch operation. If theinstruction has not been decoded, the run method causes execution of theprogram code needed to fetch the instruction, decode the instruction,and update the corresponding entries in table 24. Instruction frame 18Aalso contains next object pointer 22, which indicates a next proxyobject in the linked list to which to pass the request after processingby instruction frame 18A is complete.

Referring now to FIG. 3B, an architecture of check frame 18B of FIG. 2Bis shown in accordance with an embodiment of the present invention.Check frame 18B contains a table 20 with special handling entries of theform:

Location Type Handler Address1 Breakpoint &breakpoint_handler Address 2Instruction memory watch &watch_handler Address 3 Exception&exception_handlerCheck frame 18B further contains a next object pointer 22, which can beplaced in an upstream object's next object pointer when check frame 18Bis removed from a chain of special handling frames, since the chain ofspecial handing frames forms a linked list with next object pointer 22providing the link. The above is only an example of a data structurethat can be used within check frame 18B to represent special handlingoperations and other forms of information that can provide some or allof the above-described capabilities can alternatively be implemented byframe proxy 16.

Referring now to FIG. 4, a method of managing memory accesses inaccordance with an embodiment of the invention is exemplified. Theparticular method is a simulation method. The simulation is started(step 30) and when a fetch for a next instruction to execute is received(step 31), if the current frame being processed is an instruction frame(decision 32), if a decoded instruction for the program address is notcached (decision 33), then the instruction is retrieved from simulatedinstruction memory and decoded (step 34). Once the decoded instructionis available, the decoded instruction is simulated (step 35). If thenext instruction is decoded and cached (decision 36), steps 35 and 36are repeated directly without requiring exit of the instruction frame.If the current frame was not an instruction frame (decision 32), if thecurrent frame is a check frame implementing a breakpoint, code watch, orexception entry for the code location (or range containing the codelocation) (decision 37), then the corresponding handler is called ortriggered (step 38). Until the last frame in the chain (linked list) isencountered (decision 39), steps 32-39 are repeated and until thesimulation is complete (decision 40) steps 31-39 are repeated.

Referring now to FIG. 5, a computer system in which the above-describedcomputer-performed methods are performed, is shown. A general-purposecomputer 100 is coupled to a graphical display 102 for display ofsimulation results, as well as user interface screens for controllingone or more programs including programs forming computer programproducts including program instructions for carrying out the methodsdescribed above. Input devices such as a keyboard 108A and a mouse 108Bprovide for user interaction with general-purpose computer 100.General-purpose computer 100 includes a processor 104 for executingprogram instructions stored in a memory 106 including programinstructions forming the above-described computer program products inaccordance with embodiments of the invention. A removable storagedevice, such as a DVD-ROM drive 105 is included for accepting storagemedia such as DVD-ROM DVD1 forming a computer program product inaccordance with an embodiment of the invention. The computer systemdepicted in FIG. 5 is only one example of a computer system, such as adesktop computer system or a notebook computer system, which can be usedto perform the verification described above. Other types of computersystems, including distributed and networked computer systems in whichsome or all of the algorithms and portions thereof are executed remotelyare within the scope of the invention as contemplated herein. The systemdepicted in FIG. 5 should not be construed as limiting the type ofcomputer system or computer program products that form embodiments ofthe present invention.

As noted above, portions of the present invention may be embodied in acomputer program product, which may include firmware, an image in systemmemory or another memory/cache, or stored on a fixed or re-writablemedia such as an optical disc having computer-readable code storedthereon. Any combination of one or more computer-readable medium(s) maystore a program in accordance with an embodiment of the invention. Thecomputer-readable medium may be a computer-readable signal medium or acomputer-readable storage medium. A computer-readable storage medium maybe, for example, but not limited to, an electronic, magnetic, optical,electromagnetic, infrared, or semiconductor system, apparatus, ordevice, or any suitable combination of the foregoing. More specificexamples (a non-exhaustive list) of the computer-readable storage mediumwould include the following: an electrical connection having one or morewires, a portable computer diskette, a hard disk, a random access memory(RAM), a read-only memory (ROM), an erasable programmable read-onlymemory (EPROM or Flash memory), an optical fiber, a portable compactdisc read-only memory (CD-ROM), an optical storage device, a magneticstorage device, or any suitable combination of the foregoing.

In the context of the present application, a computer-readable storagemedium may be any tangible medium that can contain or store a programfor use by or in connection with an instruction execution system,apparatus, or device. A computer-readable signal medium may include apropagated data signal with computer-readable program code embodiedtherein, for example, in baseband or as part of a carrier wave. Such apropagated signal may take any of a variety of forms, including, but notlimited to, electro-magnetic, optical, or any suitable combinationthereof. A computer-readable signal medium may be any computer-readablemedium that is not a computer-readable storage medium and that cancommunicate, propagate, or transport a program for use by or inconnection with an instruction execution system, apparatus, or device.Program code embodied on a computer-readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:
 1. A computer system comprising a first processorfor executing program instructions coupled to a memory for storing theprogram instructions, wherein the program instructions are programinstructions for processing simulated accesses to instruction memory bya second processor being simulated by a simulation program executingwithin a simulated computer system, and wherein the program instructionscomprise: program instructions for receiving, from the simulatedprocessor, a next simulated instruction access request to access aninstruction memory location within a frame of the simulated memory at aproxy frame corresponding to the instruction memory frame, wherein theinstruction memory frame represents a tail of a linked list accessed viathe proxy frame; program instructions for, if an instruction frame hasbeen inserted in the linked list ahead of the memory frame, processingthe next simulated instruction access request by the instruction frame,wherein the instruction frame provides a buffered decoded instruction tothe simulation program; and program instructions for if the instructionframe has not been inserted ahead of the memory frame, processing thenext simulated instruction access request by the memory frame directly.2. The computer system of claim 1, further comprising inserting theinstruction frame between the proxy frame and the instruction memoryframe upon detecting a first access to the instruction memory frame. 3.The computer system of claim 1, wherein the processing of the nextsimulated instruction access request comprises fetching an instructionstored at the instruction memory location, and wherein a result ofprocessing the next simulated instruction access request is stored inconformity with a result of the fetching, whereby subsequent processingof another simulated instruction access request to the same instructionmemory location by the proxy frame does not require fetching theinstruction from the memory subsystem.
 4. The computer system of claim3, wherein the processing of the next simulated instruction accessrequest further comprises decoding the instruction, and wherein a resultof the decoding is stored by the instruction frame, whereby subsequentprocessing of the another simulated instruction access request by theproxy object does not require decoding the instruction.
 5. The computersystem of claim 1, wherein the processing of the next simulatedinstruction access request comprises fetching and decoding aninstruction stored at the instruction memory location, and wherein aresult of the decoding is stored by the instruction frame, wherebysubsequent processing of another simulated instruction access request bythe frame proxy does not require fetching the instruction from thememory subsystem and decoding the instruction.
 6. The computer system ofclaim 5, wherein the processing of the next simulated instruction accessrequest further comprises storing a pointer to a next instructionlocation corresponding to a next instruction previously executed afterthe instruction, whereby completion of processing of the instructiondoes not require determining a next instruction to execute.
 7. Thecomputer system of claim 1, wherein the receiving receives the nextsimulated instruction access request at a chain of multiple command andcheck frames including the instruction frame and one or more other checkframes, and wherein each of the one or more check frames performs aseparate check on the instruction memory access request.
 8. A computerprogram product comprising a computer-readable memory that is not asignal or carrier wave storing program instructions for processingsimulated accesses to instruction memory by a simulated processor beingsimulated by a simulation program executing within a simulated computersystem, wherein the program instructions comprise program instructionsfor: receiving, from the simulated processor, a next simulatedinstruction access request to access an instruction memory locationwithin a frame of the memory of the computer system at a proxy framecorresponding to the instruction memory frame, wherein the instructionmemory frame represents a tail of a linked list accessed via the proxyframe; if an instruction frame has been inserted in the linked listahead of the memory frame, processing the next simulated instructionaccess request by the instruction frame, wherein the instruction frameprovides a buffered decoded instruction to the simulation program; andif the instruction frame has not been inserted ahead of the interfaceand the memory frame, processing the next simulated instruction accessrequest by the memory frame directly.
 9. The computer program product ofclaim 8, further comprising inserting the instruction frame between theproxy frame and the instruction memory frame upon detecting a firstaccess to the instruction memory frame.
 10. The computer program productof claim 8, wherein the processing of the next simulated instructionaccess request comprises fetching an instruction stored at theinstruction memory location, and wherein a result of processing the nextsimulated instruction access request is stored in conformity with aresult of the fetching, whereby subsequent processing of anothersimulated instruction access request to the same instruction memorylocation by the proxy frame does not require fetching the instructionfrom the memory subsystem.
 11. The computer program product of claim 10,wherein the processing of the next simulated instruction access requestfurther comprises decoding the instruction, and wherein a result ofdecoding is stored by the instruction frame, whereby subsequentprocessing of the another simulated instruction access request by theproxy object does not require decoding the instruction.
 12. The computerprogram product of claim 8, wherein the processing of the next simulatedinstruction access request comprises fetching and decoding aninstruction stored at the instruction memory location, and wherein aresult of the decoding is stored by the instruction frame, wherebysubsequent processing of another simulated instruction access request bythe frame proxy does not require fetching the instruction from thememory subsystem and decoding the instruction.
 13. The computer programproduct of claim 12, wherein the processing of the next simulatedinstruction access request further comprises storing a pointer to a nextinstruction location corresponding to a next instruction previouslyexecuted after the instruction, whereby completion of processing of theinstruction does not require determining a next instruction to execute.14. The computer program product of claim 8, wherein the receivingreceives the next simulated instruction access request at a chain ofmultiple command and check frames including the instruction frame andone or more other check frames, and wherein each of the one or morecheck frames performs a separate check on the instruction memory accessrequest.